vapor pressure and liquids

1. 你在 google 打 " vapor pressure of ethanol " 和
" vapor pressure of methanol " 便可以在維基看到圖表。methanol 的 vapor pressure 會較高。何解呢？

從液態化為氣態的分子愈多，vapor pressure 便會愈大，這受两個因素 factor 影响。

1. 分子間引力(intermolecular force), the stronger the intermolecular force, the lower the vapor pressure.

2. Molecular mass, the higher the molecular mass, the lower its vapor pressure.

Intermolecular force of methanol and ethanol are the same kind. They are both hydrogen bonding.

Molar mass of methanol is lower than ethanol. Methanol is easier to escape from its liquid than ethanol and hence results in a higher vapor pressure.

2. Clausius-Clapeyron plot of superimposed data

ln(P) = -∆Hvap/(RT) + C

Compare their slope, the most negative one has the highest enthalpy of vaporization.

3. The more positive the enthalpy of vaporization ( endothermic), the stronger the intermolecular force and lower the vapor pressure.

4. which one in each pair that would have the higher enthalpy of vaporization.

a. hexanol or hexane Answer:__hexanol__ (stronger intermolecular force, hydrogen bonding)

b. propanol or butanol Answer:__butanol__ (higher molar mass)

c. dimethyl ether or ethanol Answer:__ethanol__ (stronger intermolecular force, hydrogen bonding)

d. pentanoic acid or pentanal Answer:__pentanoic acid__ (It forms dimer and hence effectively double its molecular weight.)

5. H2O(l) → H2O(g) (100°C) ΔHm = 40.67 kJ mol⁻¹

Methanol is 35.21 kJ mol⁻¹

Water is higher because it can form more no. of hydrogen bonding per molecule. each water molecule carry 2 hydrogen.

6. "If the experiment was started with a wet flask," What experiment ? 睇唔明題目。Sorry!